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in                             SKULL,   OF   CYNOGJSTAT&US                            93
and in Diademodon for instance. This incidentally lends some
support to the idea that the Mammalia have been evolved from
two sources, a way of looking at the origin of the group that
will coincide with the views of some authors like the late Dr.
Mivart, and will at the same time reconcile the trituberculists
and the multituberculists. For we should then assume that
the Eutheria and Triconodontia had originated from some such
form as OynognatJius; and the Multituberculata and the ex-
isting Monotremes from some form like Diademodon. It is not
of great use to point out that Diademodon is really of the tri-
tuberculate pattern, because in its molars, though multituberculate,
the trituberculate main cones can be recognised; for that state
of affairs could just as well have been brought about by a
reduction from the multituberculate type. The skull of these-
Theriodonts shows some well-marked approximations to the
mammalian type. There is in the first place a commencing
consolidation and reduction of the individual bones, which is so
distinguishing a feature of the mammalian skull as opposed to the
skull of lower vertebrates. In Cynognathus the postorbital is
fused with the jugal, and the supratemporal with the squamosal,
forming apparently one bone. In the lower jaw the splenial is
often reduced to the thinness of paper, thus indicating a com-
mencing disappearance. In many Theromorpha the squamosal
shares largely in the formation of the articular facet for the
lower jaw, obviously an important mammalian characteristic ;
this is brought about by the reduction of the quadrate, which
latter bone, moreover, acquires in certain particulars the appear-
ance of the mammalian malleus, with which it is, according
to many, homologous. But this subject has been already dealt
with on page 26. A-very pronounced likeness to the mam-
malian skull is that there are two occipital condyles. That
this has been brought about "by the further development of
a tripartite condyle such as occurs in tortoises, and that by
the suppression of the basi- occipital part, does not affect the
resemblance to the mammalian skull; in fact it explains the
origin of two condyles from the typical reptilian single con-
dyle, and disposes of the necessity for believing, with Huxley
and others, the Amphibia to be on the main line of mani-.
malian evolution on account of their two condyles. The
general aspect of the skull in Cynognathus has been com-